What it does do is put a really freaking big and powerful radar array in Korea, that is so powerful it can actually track all the way into China. FWIW China hates this because it basically gives the U.S. a "leg up", in a theoretical nuclear exchange because we have earlier detection capability than China does versus us now.

The odd thing is that China has made comparatively much less fuss about the United States selling Pave Paws to Taiwan, a radar system which sees far deeper into China than THAAD does.